Simultaneous Measurement of Condensation and Thermal Accommodation Coefficients

University Of Utah, Salt Lake City UT

Investigators

Abstract

As a cloud droplet grows by condensation, water molecules flow from the vapor surrounding the droplet to its surface and the heat released by condensation flows away from the droplet. Classical theory treats the environment of the drop as a continuum, so that the heat and mass transfer are described by the diffusion equation. This is a valid approximation for drops that are much larger than the molecular mean free path, which is about 0.06 mm for normal sea level conditions. However, newly formed cloud droplets have sizes typically between 0.1 and 1 mm, for which the classical, continuum theory must be modified to include gas kinetic effects. The modification introduces two new parameters that characterize the transfer of heat and mass, the accommodation coefficient and the condensation coefficient, denoted respectively by a and b. The accommodation coefficient may be thought of as the fraction of the molecules bouncing off the surface of a drop that have acquired the temperature of the drop. The condensation coefficient may be described as the fraction of the molecules hitting the surface of the drop that stick to it. In general, a and b are not equal and must be determined experimentally, because there is no known way to derive their values theoretically. Though there have been previous attempts to determine the coefficients, the results are quite variable. There are several reasons for this. First, it is no easy matter to measure the rate of growth of a micron-sized droplet in controlled conditions. Second, a and b are linked in such a way that a given droplet growth rate can be explained by many combinations of a and b. Third, there is evidence that the values of the coefficients may be influenced by small amounts of impurities in the water. This project aims to determine a and b separately by taking advantage of their different dependence on atmospheric pressure. The rates of growth of small droplets are observed under different conditions of temperature and pressure by illuminating them with a laser and employing the known peaks in the Mie scattering curve for water droplets to determine drop size. Experiments with drops formed on aerosols of different chemical composition will indicate the possible influence of contaminants on a and b. There is already some evidence that impurities lower the values of these coefficients. The effect of lower values is to reduce the growth rates of small droplets. A consequence of smaller droplet size is a greater reflectivity of the cloud for solar radiation. Therefore pollution aerosols may have the effect not only of increasing the number of small cloud droplets but also retarding their early growth by condensation. The correct treatment of the effects of clouds on solar radiation may therefore require accurate knowledge of the accommodation and condensation coefficients.